The present invention relates to methods for fabricating a nozzle box assembly for a steam turbine and particularly relates to a process for welding component parts of a nozzle box assembly to one another with resulting reductions in cycle time and manufacturing costs.
A nozzle box assembly for a steam turbine is essentially comprised of three components: a torus, a bridge ring and a nozzle. These major components are very high strength very high temperature resistant forgings. Each of the components is initially formed in 180xc2x0 segments. After the components are assembled (welded) to form two (upper and lower) unitary 180xc2x0 nozzle box halves, the halves are joined one to the other along a horizontal midline to form a complete annular nozzle box assembly for a steam turbine. Each completed nozzle box half includes one or more, preferably two, steam inlets formed integrally with the torus. These inlets extend from the torus in a plane normal to the axis of rotation of the turbine and hence lie close to and in the plane of the torus itself. In a completed turbine, the inlets receive steam from a suitable source for flow into the torus. The steam changes direction to a generally axial flow for flow through the annular opening of the bridging ring and into the nozzles of the nozzle plate which include air foil vanes for directing the steam flow to subsequent buckets. It will be appreciated that the bridge ring is welded, as discussed below, to the torus about the radially inner and outer margins of the torus. The bridge ring serves as a buttress to hold the margins of the torus together against the very high pressures of the steam tending to separate the margins from one another. The nozzle plate is in turn welded to the bridge ring on a side thereof remote from the torus.
In a prior manufacturing method for forming the nozzle box assembly, four major welds were required. The first and second welds were formed on the inner and outer radius between margins of the torus and the bridge ring to secure those parts to one another. Third and fourth welds were also formed between the bridge ring and nozzle plate along respective inner and outer radii to secure those parts to one another. The bridge ring was originally provided with extra metal along a side thereof remote from the torus. This extra metal not only closed off the opening in the forged torus but afforded extra mass to minimize thermal distortion during the welding process. The extra mass of metal was later machined from the bridge ring opening up the semi-circular opening from the torus through the bridge ring.
Initially, the bridge ring with the extra metal mass was fitted up to the torus on tapered surfaces,that registered the bridge ring to the torus. Prior to fitting up, however, weld prep was performed on each of the margins of the bridge ring and torus along inside and outside radii and inspected to insure that the weld prep was flat, level and true and met the requirements for the weld. Once the bridge ring and torus were assembled and clamped to one another, this subassembly was preheated to approximately 450-500xc2x0 F. The preheat temperature was monitored to ensure the components obtained the desired temperature. Once the weld prep was validated and the torus and bridge ring were preheated, a small diameter electrode TIG welding process was used to consume the abutting portions of the bridge ring and torus in a root pass. That is, the first root pass melted the adjoining metal such that the two components became integral without any separation line thus forming a homogeneous piece. Additional passes by a hand TIG welding torch added further material which formed base material for the final weld. Once there was significant weld buildup, the welding process was changed to a submerged arc process for high weld deposition where weld buildup was effected quickly. The preheat temperature was maintained throughout the welding process. The welded assembly was immediately placed into a stress relieving oven for a predetermined time frame without allowing the materials to cool. Specifically, the welded torus and bridge ring were placed in an inert gas furnace and the temperature was raised to 1200xc2x0 F. and above with a subsequent controlled cooling rate. This completed and cooled subassembly was then X-rayed to insure the integrity of the weld.
Assuming the weld passed the X-ray test, the extra metal mass of the bridge ring was machined thus machining off the heat sink and opening the torus. The torus-bridge ring subassembly and nozzle plate were then welded to one another along inner and outer radii utilizing a similar procedure. For example, weld prep for the bridge ring and nozzle plate was performed and validated and the components were aligned and clamped to one another. The clamped assembly was preheated to approximately 450xc2x0 F. and a first root pass was made by TIG welding to consume the root and form a homogeneous assembly. An additional three or four passes using a hand-held TIG torch built up the base metal and that welding process was followed by a submerged arc welding process which rapidly added additional weld material. The assembly was stress relieved, X-rayed and machined to finish.
Apart from time and labor costs, the foregoing described process experienced unique problem associated with the weld prep and use of the welding equipment for joining the torus and bridge ring. The inlet snouts on the torus included projections which typically Interfered with the welding process. Particularly, the projections did not permit access of the welding tools to the radial outer weld between the bridge ring and torus. To afford access, a portion of the inlet snout material was removed to enable welding between the bridge ring and torus. Once the welding was complete, the previously removed snout material was restored to the torus by welding. This removal and later restoration of material involved substantial time and labor costs.
The foregoing described welding procedures to form the nozzle box assembly for a steam turbine were thus extensive, time consuming and very costly. The welds between the torus and bridge ring and between the nozzle plate and the bridge ring were necessarily performed at different times, the latter being performed only after completion of the former. As a consequence, the prior assembly procedure required four weld preps, two preheats, four welds, two stress relieves, two X-rays and twice the heavy-duty machining to produce the final nozzle box assembly. As a result, the fabrication of the nozzle box assembly often required approximately one year""s time and used highly skilled welders and machinists.
In accordance with a preferred embodiment of the present invention, substantial time and hence cost savings are realized in the fabrication of a nozzle box assembly by utilizing only two major welds necessitating only two weld preps, one preheat, one stress relief, one X-ray and one heavy-duty machining in order to assemble the torus, bridge ring and nozzle plate and form the final nozzle box assembly. The two major welds are along the inner radius and outer radius, respectively, and directly join the torus, bridge ring and nozzle plate to one another to form a unitary homogenous assembly. Additionally, the step of removing the extra mass of metal from the bridge ring subsequent to welding the bridge ring to the torus and prior to its securement to the nozzle plate is entirely eliminated. Also, the torus is modified about the margins of its inner and outer radius to provide an inwardly directed face or turn at each inner and outer radius. Thus, the torus prep area tapers inwardly along the margins toward the torus opening. The thickness of the bridge ring is reduced and the previously employed extra mass of metal material is entirely eliminated. The bridge ring is provided with robust positioning fits along opposite annular margins to fit with the torus and the nozzle plate respectively. Additionally, the outer radius of the nozzle plate adjacent the bridge ring has a substantially increased taper to provide additional welding access in the single outer weld prep area adjacent the snout of the torus. This eliminates the heretofore necessity of removing snout material to gain access for weld prep and the welding tools and then restoring the material once the welds have been completed.
To fabricate the nozzle box assembly in accordance with a preferred embodiment of the present invention, weld preps are performed on the nozzle plate and margins of the torus. The bridge ring is fitted to the torus about the opening and the nozzle plate is fitted to the bridge ring with precision fits. The assembly is then clamped tightly to one another in preparation for welding. The welding procedure is similar to the previously described welding procedure with respect to the prior nozzle box assembly except that only two major welds are required rather than four. Thus, once clamped, the three assembled components are preheated to approximately 450xc2x0 F., root weld passes are performed to join the metal at the fit ups and additional passes are made to build up weld material. Once significant weld buildup is present, a submerged arc TIG welding process is employed to enable fast weld deposition. Once welding is completed the assembly is stress relieved without first cooling, and subsequent X-ray examination and final machining are performed.
In a preferred embodiment according to the present invention, there is provided a method of manufacturing a nozzle box assembly half for a turbine comprising the steps of (a) providing arcuate segments of a torus, a bridge ring and a nozzle plate, (b) clamping the torus and bridge ring segments to one another and the bridge ring and nozzle box segments to one another with the bridge ring segment between the torus and nozzle plate segments and (c) welding along inner and outer radii of the assembly and overlying the bridge ring segment to join directly the nozzle plate, bridge ring and torus segments to one another with the bridge ring segment therebetween.
In a further preferred embodiment according to the present invention, there is provided a method of manufacturing a nozzle box assembly half for a turbine comprising the steps of (a) providing semi-circular segments of a torus, a bridge ring and a nozzle plate, (b) fitting the torus and bridge ring segments to one another and the bridge ring and nozzle box segments to one another with the bridge ring segment between the torus and nozzle plate segments and (c) applying weld material along inner and outer radii of the assembly and overlying the bridge ring segment to directly join the nozzle plate, bridge ring and torus segments to one another with the bridge ring segment therebetween.